If I write a math book, and you read it, then tell someone about the math within it. You are not violating copyright. In fact, you could write your OWN math book, or history book, or whatever, and as long as you're not copying my actual text, you are not violating copyright.
However, when an LLM does the same, people now what it to be illegal. It seems pretty straightforward to apply existing copyright law to LLMs in the same way we apply them to humans. If the actual text they generate is substantially similar to a source material that it would constitute a copyright violation if a human were to have done it, then it should be illegal. Otherwise it should not.
edit: and in fact it's not even whether an LLM reproduces text, it's wether someone subsequently publishes that text. The person publishing that text should be the one taking on the legal hit.